Current techniques for measuring thermophysical properties, particularly thermal conductivity, in high-temperature, in-pile applications are both slow and inaccurate. For instance, thermophysical properties must be known before new fuel compositions and structural materials are deployed in nuclear reactors. Thermal conductivity is one of the most important properties for predicting fuel and material performance, and is highly dependent on physical structure, chemical composition, and state of matter. During irradiation, the physical structure and chemical composition of nuclear fuels and components change as a function of time and position within the reactor.
Measurement of thermal conductivity of nuclear fuels and materials is currently primarily done in “hot cells.” In these cells, previously-irradiated samples are removed from their environment for testing. This technique has several disadvantages: it is expensive and time consuming to repeatedly remove and return samples to the in-pile testing environment, the process may disturb the physical properties of the sample, and this method can only provide a snapshot of the sample's physical properties at the end state when the measurement is made.
Currently, a thermocouple approach is the only technique used to detect thermal conductivity in high-temperature reactor applications (i.e. in-pile testing of nuclear fuel). Typically this approach uses thermocouples inserted into the interior and exterior of a sample. This approach assumes several conditions about the sample: uniform composition, uniform density, minimal gap conductance effects, and uniform heat generation. Additionally, this method of testing requires specially designed samples to minimize these factors. Hence, the current approach requires specialized (non-prototypical) samples and is susceptible to high levels of uncertainty due to the assumptions made. Therefore, a need for an accurate way of measuring thermal conductivity without removing samples is needed.
Additionally, in nuclear applications, transmutation of elements can alter the performance of probe materials as well as potentially cause damage to probe components. For example, tungsten and rhenium thermocouples can be decalibrated by transmutation in-pile. Thus, there is a significant need for probes with both temperature and radiation resistance.